Built-in antimicrobial plastic resins and methods for making the same

ABSTRACT

Provided herein is a method for preparing antimicrobial thermoplastic resins and products thereof.

CROSS REFERENCE TO RELATED APPLICATION

This is a continuation patent application of U.S. non-provisional patent application Ser. No. 15/382,792, filed on Dec. 19, 2016, which is a continuation-in-part patent application of U.S. non-provisional patent application Ser. No. 15/318,585 filed on Dec. 13, 2016, which is a national phase patent application of PCT application number PCT/US2016/070500 filed on Jan. 8, 2016 claiming benefit from U.S. provisional patent application Ser. No. 62/124,973 filed on Jan. 9, 2015, and the disclosure of which are incorporated herein by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to antimicrobial plastic resins and the methods for making the same, in particular, relates to built-in, biocide-free and safe-to-use antimicrobial plastic resins and the methods for making the same.

BACKGROUND

Despite the biocidal actions to kill the bacteria after attachment to the substrate through several mechanisms, such as inhibition of the synthesis of the cell wall, the nucleic acid, the protein and finally the distortion of the cellular metabolism (Murray et al. Medical Microbiology, 4^(th) ed.; Mosby: St Louis, 2002), there are other more natural microbial prevention strategies that can be applied onto the polymer surfaces. While there might be others, one very good example of demonstration is the inherent antimicrobial property of chitosan, a natural polysaccharide derived from the shrimp (Chung et al. Acta Pharmacol. Sin. 2004, 25, 932). The most common antimicrobial products adopt bacteria killing as the strategy through biocidal actions and therefore the bacterial prevention effectiveness heavily depends on the release and replenishment of the leachable biocides that are required to migrate to the surfaces from the inner matrix. The biocides must dissipate off the surface for killing a microbe, leaving behind less antimicrobial effectiveness for future microbial encounters. This means that the microbial protection is perishable and has a fixed shelf life. Furthermore, over extended time, the adsorbed microbes on the substrate will gradually adapt to the surfaces, if the biocidal action is not immediate or if biocides are underdosed at levels below the minimum inhibitory (MIC) or minimum bactericidal concentrations (MBC). Each bacterial isolate has its specific MIC or MBC. This aids to select biocide-resistant microbes.

WO2005021626 discloses a built-in antimicrobial formulation for acrylic polymers by mixing with several organic antimicrobial additives in articles.

US20140008324 discloses methods for processing plastic substrates, comprising at least one of injection molding, thermoforming, or extruding, having inorganic antimicrobial microparticles within and then using plasma etching which results in the removal of a portion of the substrate surface and thereby exposing material within the substrate.

Photocatalysts, such as ZnO and TiO2, exhibit disinfecting effects against gram-positive and gram-negative bacteria and they work only under UV irradiation (Dhanalakshmi et al. Mater. Express 2013, 3, 291).

Others employ electrostatic method by the use of cationic polymers, such as chitosan, a natural polysaccharide. The bactericidal action of chitosan targets at the cell membrane of the bacteria which is negatively charged. The bacteria preferentially adsorb to the polymer surface and the strong adhesion leads to a gradual increase in their cell permeability and eventually intracellular dissolution due to the distortion of the charge distribution of the membrane (Chang et al. J. Agric. Food Chem. 2012, 60, 1837). However, this approach is not universal to target at a broad spectrum of bacteria bearing different cell membrane charges; some bacteria can be positively charged.

On the other hand, the lower-molecular weight polymers of chitosan were found to be able to diffuse and permeate through the porous membrane into the cells and form stable complexes with DNA. This subsequently prevents the DNA transcription activities, thus leading to inhibition of the proliferation and even the death of bacteria (Kenawy et al. Biomacromolecules 2007, 8, 1359).

Another biocidal approach requires complex nanoscale features and pattern topographies, such as nanopillars, which can be found on the insect wings (Pogodin et al. Biophys. J. 2013, 104, 835).

DE19535729 discloses a biocide-free coating free from biocides based on organofunctional silanes and fluoroorganosilanes and/or their hydrolysates and/or condensation products but the invention is limited to coatings which function properly with metal substrates, such as aluminum foil.

All these cater for the development of the new approach via a bacteria-repellent layer to be permanently and stably formed on the surface of a commodity plastic article and to prevent adhesion and accumulation of bacteria. This overcomes all the drawbacks that come with the conventional surface coating and/or combined with biocidal approaches.

One feasible antifouling structure is based on electrostatics via charge-bearing polymers, polyelectrolytes, polysaccharides and polypeptides containing amino, quaternized, carboxylated, sulfonated, phosphate, boronate entities and other metal oxides, complexes and their derivatives, of which the zeta potentials can be finely tuned by pH, counterion and charge valence.

U.S. Pat. No. 8,545,862 discloses an anionic/cationic polyelectrolyte complex, for example, a composition consisting essentially of a derivative or copolymer of poly(acrylic acid) or polystyrene sulfonate, to impart antimicrobial properties to an article.

WO2012065610 discloses a long-lasting antimicrobial coating for fabrics comprising a polymeric quaternary ammonium (quat).

EP2627202 discloses an antimicrobial peptide comprising Brad or an active variant thereof as a food preservative to prevent or inhibit spoilage of a foodstuff by a microorganism.

Just this approach, however, is not versatile enough. It is because some bacteria, plasma proteins and red blood cells carry negative charges on their cell membranes and they are expected to show very similar electrostatic repellent behavior against a substrate surface of the same charge. While other species of bacteria can be positively charged, such as Stenotrophomonas maltophilia, this approach is therefore not universal to target at a broad spectrum of bacteria as differentiated by the cell membrane charges.

The second feasible structure of antifouling groups is derived from neutral polymers, such as poly(2-hydroxyethyl methacrylate) (polyHEMA), poly(ethylene glycol) (PEG) and Zwitterionic polymers and a heterogeneous polymer system of mixed charges comprising of cationic and anionic functionalities onto the plastic surface (Sin et al. Polym. J 2014, 46, 436). PolyHEMA shows the repellent properties because of its strong hydrophilicity so that it can displace the deposition of bacteria by a tightly bound hydration layer. Hydrophilic surfaces are apparently helpful to avoid bacterial adhesion. PEG however uses the steric exclusion effect to resist the protein and platelet adsorption. Previous data also suggested that the adherence of the bacteria was determined by the composition and the chemical nature of the pre-adsorbed protein layers coupled with the surface hydrophilicity. Zwitterionic polymers are bioinspired from the Zwitterionic phospholipid structures of the cell membranes which are well-known to be bio-inert. As different from the hydrophilic polymers, the betaine-based Zwitterionic polymers can finely tune the electrostatic interactions with the nearby water molecules and control the non-specific protein adsorption. Siedenbiedel et al. reveal successful examples of applications of Zwitterionic polymers for prevention of bacterial adhesion to the surfaces after chemical modification (Siedenbiedel et al. Polymers. 2012, 4, 46).

In the third feasible approach, the antifouling property of the plastic material can be achieved by modifying the chemical group functionality which in turn changes the surface hydrophobility via end termination and/or grafting of a polymer chain with alkyl, hydroxyl, fluoroalkyl and/or aromatic entities (Nie et al. J. Mater. Chem. B 2014, 2, 4911), completely different adhesion and physicochemical behaviors of biomolecules onto the surfaces will be acquired.

SUMMARY OF THE INVENTION

Conventional methods of fabrication of antifouling and bacteria-repellent surfaces are achieved mostly by surface chemical grafting, vacuum deposition, in-situ polymerization, spin-coating and dip-coating of related antifouling materials which have low throughputs. Another technical problem with surface modification is the prerequisite of post-treatment, such as thermal curing, photoreactions, etc. in order to ensure a uniform coverage of the coating on the articles whereas the shape limitations with curved, inner lumen and fine features are often encountered for different finished consumer products. While commodity plastic products are mainly manufactured by thermoforming processes, injection molding and extrusion in a large-scale and continuous mode of operation in plastic industries, any add-on surface modification and/or post-treatment processes to the existing production lines are certainly not desirable to manufacturers in consideration of the new capital investment.

The objective of this invention is to develop biocide-free resins overcoming the shortcomings of limitation to surface coatings or leaching out in existing technologies. Neither bactericidal nor bacteriostatic, the biocide-free resins' surface shows built-in bacteria repellent performance rather than killing performance after product forming.

The present invention is related to a reformulated thermoplastic resin with built-in bacteria-repellency and adaptability to thermoforming processes. The nonfouling functionalization of a base polymer is performed by reactive melt extrusion of a commercial thermoplastic resin, such as maleic anhydride bearing polyethylene, with an appropriate antifouling coupling agent comprising of one or more similar bio-repellent structures in the said chemical approaches and/or other free-radical initiators and acid/base catalysts, such as sodium periodate, azobisisobutyronitrile, benzoyl peroxide, dicumyl peroxide, potassium persulfate, p-toluenesulfonic acid, 4-dimethylaminopyridine, stannous chloride, dibutyltin dilaurate and trimethylsilyl chloride, thus leading to a masterbatch resin after cooling and pelletization. The resin is then molded into finished articles which display antimicrobial performance after a thermoforming process. The incorporated antifouling groups of the invention can phase segregate and inhabit the surfaces during article forming processes, hence introducing permanent barriers against bacterial adhesion via a chemically-stable repellent surface on the neat plastic article. The thermoplastic resin includes but not limited to the family of polyolefin, polyether, polyvinyl, polyester, polyacetal, polyamide, polyurethane, polyacrylate, polycarbonate, polyimide, polyphthalate, polysulfone, polythioether, polyketone, epoxide and other elastomeric polymers, such as silicone, polyisoprene, acrylonitrile butadiene styrene and ethylene vinyl acetate. Because of the low-cost and high-volume production capability using thermoforming processes, typically melt extrusion and injection molding, the formulation is melt-processable and exhibits strong thermal chemical stability up to a temperature as high as 350° C. Moreover, it tends not to interfere with the manufacturer's production line and the physical bulk properties, such as optical transparency, thermal conductivity, mechanical stiffness, electrical conductivity, dielectric strength and flammability rating, of the formed articles comprising of the formulation. The articles can be in the form of composite, fiber, sheet and varnish.

After thermoforming, a non-stick bacteria-repellent layer of polymer brush on the article surface is formed by heterogeneous phase separation, crystalline ordering and/or surface-directed migration of the backbone-grafted antifouling groups of the present invention to minimize the initial physical or chemical adsorption of motile planktonic bacterial cells and later colonization of the harbored cells leading to formation of irreversible biofilms that can withstand host-defense measures or any antibiotics in use. The tethered layer has covalently been tied with the neat matrix after article forming. This provides unperishable protection of the surface from bacterial attachment. The said brush layer is not bactericidal or bacteriostatic. This means that the mechanism of action for microbial prevention is not achieved by killing or growth inhibition of bacteria. No leachable and non-biodegradable biocidal additives are involved to move to the surface to work. This can entirely eliminate the chance to generate biocide-resistant bacteria. The invention is therefore safe-to-use in the way that the antifouling layer is biocompatible, non-cytotoxic and does not lead to either skin allergies on contact or persistent bioaccumulation in eco-system.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the preparation process of an embodiment of the present invention.

FIG. 2 is an ATR-FTIR spectrum of PEG-bearing styrene-maleic anhydride copolymer (SMA-PEG) and styrene-maleic anhydride copolymer (SMA).

FIGS. 3a and 3b are a TGA and DSC thermogram, respectively, of an antimicrobial PP resin (PP/SMA-PEG) using SMA-PEG as masterbatch.

FIGS. 4a and 4b are a TGA and DSC thermogram, respectively, of an antimicrobial PC resin (PC/SMA-PEG) using SMA-PEG as masterbatch.

FIG. 5 is an ATR-FTIR spectrum of PEG-bearing maleated polypropylene (PP-MA-PEG) and maleated polypropylene (PP-MA).

FIGS. 6a and 6b are a TGA and DSC thermogram, respectively, of an antimicrobial PP resin (PP/PP-MA-PEG) using PP-MA-PEG as masterbatch.

FIG. 7 is an ATR-FTIR spectrum of PEG-modified maleated olefin bearing polypropylene (PP/PP-MA-C/PEG) and maleated olefin bearing polypropylene (PP-MA-C).

FIG. 8 is an ATR-FTIR spectrum of N-(4-hydroxyphenyl)maleimide-modified PEG bearing polypropylene (PP/HPM-PEG).

FIG. 9 is a representative plate count result (100× dilution) of Escherichia coli adsorption on specimens thermoformed from antimicrobial PP resin.

FIG. 10 is a representative plate count result (100× dilution) of Escherichia coli adsorption on specimens thermoformed from antimicrobial PC resin.

FIG. 11 is a representative Escherichia coli adsorption test result on specimens injection-molded from antimicrobial PE resin.

FIG. 12 is a graphical representation of viability of HaCat cells on film specimens prepared by hot pressing of different chemicals: “Medium” refers to the film specimen with the medium only as a control; (1) and (2) refer to the film specimens with pristine PP and an antimicrobial PP resin (SMA/SMA/PEG/CA), respectively.

FIG. 13 is a flow chart showing the minimum essential medium (MEM) elution methodology described by the ISO 10993-5 standard.

DETAILED DESCRIPTION OF THE INVENTION

To illustrate the structure and advantages of the present invention, below is the detailed description of the present invention in combination with the figures and embodiments.

The present invention discloses an antimicrobial thermoplastic resin, which comprises a masterbatch and a basic plastic, the masterbatch is prepared by grafting an antifouling reagent onto an intermediate. The intermediate is prepared by grafting a reactive linker onto a base polymer backbone.

The base polymer backbone is a synthetic vinyl polymer with R groups. The R groups are linear and/or multi-armed chemical structures with homo- or hetero-substituted alkyl, alkenyl, alkynl, aryl, acyl, alkoxyl, thionyl, cyano, azo, silyl groups, halogens and/or cyclics. The antifouling reagent is a hydrogel forming polymer, constituting polyol, polyoxyether, polyamine, polycarboxylate, polyacrylate, polyacrylamide, polyvinylpyrrolidone, polysaccharide, Zwitterionic polyelectrolyte, a copolymerized system of polymer segments of mixed charges and/or an interpenetrating blend mixture of cationic and anionic polymers. Preferably, the antifouling reagent is polyethylene glycol. The reactive linker is thermally reactive and applies to ester-, oxo-, imine-, azole-, methine-, urea-, carbonate-, amide-, carbamate-, disulfide-, siloxane-directed or transition metal-based cross-coupling precursors. Preferably, the reactive linker is maleic anhydride. The basic plastic is a thermoplastic and melt-processable plastic resin including polyolefin, polyether, polyvinyl, polyester, polyacetal, polyamide, polyurethane, polyacrylate, polycarbonate, polyimide, polyphthalate, polysulfone, polythioether, polyketone, epoxide and elastomer. The antimicrobial thermoplastic resin may contain non-biocidal additives, including but not limited to catalysts, initiators, stabilizers, foaming agents, plasticizers, thickeners, lubricants, fillers, impact modifiers, anti-blocks, clarifiers, antistatics, flame retardants and/or colorants.

Referring to FIG. 1, a preparation process of the antimicrobial thermoplastic resin is shown. An intermediate is prepared by grafting a reactive linker on a base polymer backbone. A masterbatch resin is prepared by grafting an antifouling reagent onto the intermediate. The masterbatch resin is melt extruding to prepare a masterbatch and then the masterbatch is dried and pelletized after cooling. Melt compounding the masterbatch and a basic plastic to prepare the antimicrobial thermoplastic resin. Another preparation route is undergone through single-step extrusion of a dry blend mixture of base plastic and antifouling reagents. Molding into a finished article by a thermoforming process. The thermoforming process includes spinning, extrusion, injection, compression, foaming and drawing. The finished article is molded into a form including solid, monolith, tube, composite, fiber, film, sheet and varnish.

Embodiment 1

In this embodiment, therefore, poly(ethylene glycol) (PEG) was selected as the antifouling reagent to render the modified base polymers' bacterial repelling property. The masterbatches of PEG derivatives were introduced to the commercially available resins: polypropylenes (PP), polyethylenes (PE) and polycarbonates (PC). PP and PE demonstrate low melt processing temperatures while PC demonstrates high melt processing temperature.

1.1 Wet Synthesis of PEG-Bearing Styrene-Maleic Anhydride Copolymer (SMA-PEG) as Masterbatch.

SMA-PEG was prepared by grafting PEG 10,000 (Tianjin Kermel) onto the backbone of styrene-maleic anhydride (SMA) copolymer (Sigma-Aldrich, Catalog no. 442399) in acetone. In other words, 100 g of PEG 10,000 was first dissolved in 500 mL of boiling acetone to give a 20 wt % PEG solution. 3.2 g of SMA was subsequently dissolved in the PEG solution and the reaction mixture was stirred under reflux overnight. The reaction was quenched in hexane to give a white precipitate. The powdery precipitate was purified by filtration, followed by vacuum drying at room temperature. Equation 1 illustrates the synthetic route to obtain SMA-PEG.

From the attenuated total reflection-Fourier transform infrared (ATR-FTIR, Bruker Vertex 70 Hyperion 1000 with PLATINUM ATR, diamond crystal probe, DLaTGS detector) spectrum of SMA (c.f., dashed line in FIG. 2), the absorption band over 3000 cm⁻¹ corresponds to alkenyl C—H and C═C stretching modes of unsaturated aromatics, indicating the presence of styrene units. Another feature is the sharp absorption band from 2000 cm⁻¹ to 1600 cm⁻¹ which is a unique feature of cyclic anhydride, an indication of maleic anhydride in SMA. These two features are clearly absent in the ATR-FTIR spectrum of SMA-PEG (c.f., solid line in FIG. 2). The disappearance of the characteristic aromatic band (above 3000 cm⁻¹) is mainly due to the decrease of the concentration of styrene units of SMA-PEG near the surface because ATR-FTIR is a surface-sensitive characterization technique with a sampling depth of the order of tens of microns. The attenuation of maleic anhydride band indicates the complete consumption of maleic anhydride in SMA by esterification with hydroxyl end group of PEG.

1.2 Preparation of PP/SMA-PEG

PP (GD-150, Maoming Petro-Chemical Shihua Company) in powder form was selected as the base plastic for preparation of antimicrobial PP resins. 95 g of PP and 5 g of SMA-PEG were mixed thoroughly in a rotating drum mixer (Better Pak International YG-1KG). The drum mixing was performed by repeated 10 clockwise and anticlockwise rotations each for 1 minute at a speed of 60 rpm. The powdery mixture was subsequently extruded on a desktop single-screw extruder (Wellzoom C-type). The extruder has a nozzle diameter of 1.75 mm, length-to-diameter ratio of 10:1 and a maximum screw speed of 10 rpm, where the screw is driven by a 240-W motor. In the experiment, the temperature settings were 195° C. at the barrel and 200° C. at the die of the extruder while the speed of the screw was 5 rpm for extrusion. The extrudate in the form of short filaments was cooled down in air and further cryogenically granulated into powders with a swing-type stainless steel three-blade pulverizer (Laifu LFP-2500A).

The thermogravimetric (TGA, TA Q5000, at a heating rate of 5° C./min with an air flow rate of 25 mL/min) result (c.f., FIG. 3a ) on PP/SMA-PEG sample shows a peak decomposition temperature to be slightly beyond 250° C. alongside a substantial weight loss in air. Certain extent of reduction of the decomposition temperature as compared with the base PP plastic (298° C.) was expected. Although PEG is susceptible to oxidative degradation which worsens upon heating, the TGA result on PP/SMA-PEG has evidently excluded the thermal stability concern with PEG modification.

FIG. 3b shows the differential scanning calorimetry (DSC, TA Q1000) thermograms of PP/SMA-PEG resin in one heating-cooling cycle at a ramping rate of 5° C./min in nitrogen atmosphere. Two endothermic peaks were observed on the heating trace: a peak at 61° C. corresponding to the melting of PEG and a peak at 168° C. corresponding to the melting of PP, thus making it suitable for injection molding or other subsequent thermoforming processes for article formation which fits within the processing window of 30 to 50° C. above its melting temperature. The evolution of the two peaks indicates the formation of phase separated morphology of the as-extruded PP/SMA-PEG resin.

The melt flow index of PP/SMA-PEG was measured on a melt indexer (Model KL-MI-BP, Dongguan Kunlun Testing Instrument Company) to be 13.20 g/10 min at 190° C. under the load of 2.16 kg, a value higher than that of the pristine PP (GD-150) which was 9.68 g/10 min. At 230° C. under the same load, the melt of PP/SMA-PEG resin flowed like water very rapidly.

1.3 Fabrication of Antimicrobial PC Resins (PC/SMA-PEG) Incorporating Antifouling Masterbatch

PC/SMA-PEG was prepared by single-screw extrusion of a blend of PC (Teijin Panlite® L1225Y) granules (95 g) and SMA-PEG powders (5 g) which have been pre-mixed in a drum mixer by the same condition as in the foregoing sub-section 1.2. The barrel and the die temperatures for extrusion were set at 275° C. and 280° C. respectively. The speed of the screw was adjusted to 3 rpm due to the low melt viscosity of PC. The extrudate in the form of short filaments was cooled down in air and then cryogenically granulated into powders with a three-blade pulverizer.

The melt flow index of the PC/SMA-PEG resin was measured to be 21.60 g/10 min which was nearly double that of the pristine PC (11.68 g/10 min) at 300° C. under the same load of 1.2 kg.

There were two distinct decomposition peak temperatures for the PC/SMA-PEG samples: one near 400° C. and another one near 550° C. (c.f., FIG. 4a ). The first decomposition peak might correspond to the decomposition of the resin rich in SMA-PEG content. The first decomposition temperature was not reduced as much compared with that of the pristine PC which was recorded to be 445° C. Evidence of the presence of SMA-PEG was located on the DSC heating curve of the sample (c.f., FIG. 4b ), wherein an endothermic peak was shown at 60° C. This peak is assigned as the melting temperature of SMA-PEG. Since PC is intrinsically an amorphous polymer, no melting peak should be acquired for PC despite the typical second-order endothermic transition at 150° C., which is assigned as the rubber-glass transition temperature of PC. The first-order melting transition observed in PC/SMA-PEG therefore represents the SMA-PEG phase domains in the PC matrix. An additional exotherm is observed at about 327° C. attributing to some extent of bond formation events, most probably transesterification reactions between PC and the free hydroxyl ends of SMA-PEG.

1.4 Fabrication of Antimicrobial PE Resins (PE/SMA-PEG) Incorporating Antifouling Masterbatch

PE/SMA-PEG was prepared by twin-screw extrusion of a blend of PE (SABIC® 1922SF) granules (500 g) and SMA-PEG powders (25 g) which have been pre-mixed in a drum mixer by the same condition as in the foregoing sub-section 1.2 or 1.3. The extrusion was undergone on a co-rotating twin-screw extruder (Model AK26, Nanjing KY Chemical Machinery) with a length-to-diameter ratio of 44:1, a screw diameter of 26 mm and a maximum screw speed of 600 rpm. The screw rotation was driven by a 7.5 kW motor. The extruder was connected in line with a water bath followed by a pelletizer. The barrel temperature profile from the front to the rear (with a total of 8 temperature zones) was read as: 150° C., 160° C., 170° C., 170° C., 170° C., 170° C., 170° C., and 160° C. The feed frequency was 2 Hz and the speed of the screw was 150 rpm. The extrudate was solidified from melt upon cooling in water and finally subjected to pelletization. The plastic pellets were then dried in oven at 50° C. overnight.

Embodiment 2

2.1 Wet Synthesis of PEG-Bearing Maleated Polypropylene (PP-MA-PEG) as Masterbatch

PP-MA-PEG was prepared by wet chemistry via grafting of PEG 2000 (Tianjin Kermel) onto maleated polypropylene (PP-MA) (Sigma-Aldrich, Catalog no. 427845) with a weight average and number average molecular weight of 9100 and 3900, respectively, under reflux in toluene. 10 g of PP-MA was first dissolved into 200 mL of boiling toluene to give a 5 wt % solution of PP-MA. 20 g of PEG 2000 was added into the PP-MA solution. The reaction mixture was refluxed overnight and quenched in hexane to give product as a white precipitate. The precipitate was filtered to remove unreacted PEG and finally dried at room temperature in vacuum. The reaction scheme is demonstrated in Equation 2.

FIG. 5 shows the ATR-FTIR spectrum of PP-MA-PEG sample. A strong peak emerged at 1115 cm⁻¹, corresponding to the C—O stretching mode due to the ether (—O—CH₂—) linkage in PEG. On the other hand, the band typical of the unreacted maleic anhydride groups that should appear at about 1780 and 1856 cm⁻¹ for symmetric and asymmetric >C═O frequency of the C═O groups on the cyclic anhydride structure, respectively disappeared on the ATR spectrum of PP-MA-PEG. The C═O band seen between 1725 cm⁻¹ and 1500 cm⁻¹ could represent the hydrolysed maleic anhydride groups. At least one hydroxyl end group of PEG was covalently grafted to the PP backbone by esterification through ring opening of the anhydride structure and this might result in a partially converted acid form of the maleic anhydride group. The result confirms that PEG was engrafted onto PP-MA backbone and a portion of PEG chain tended to migrate to the material surface.

2.2 Preparation of PP/PP-MA-PEG

A clear grade PP (Total Petrochemicals Lumicene® MR10MX0) in granular form was selected as the base plastic for preparation of antimicrobial PP resins. PP/PP-MA-PEG was prepared by extruding a dry mixture of the PP granules and PP-MA-PEG powders on a drum mixer by the same condition. In this case, 7.5 g of PP-MA-PEG was blended with 92.5 g PP to give a total 100 g of the mixture which was subsequently extruded on a single-screw extruder. The temperature settings were 180° C. and 185° C. for the barrel and the die of the extruder and the speed of the screw was 7 rpm for extrusion. The extrudate in the form of short filaments was cooled down in air and then cryogenically granulated into powders with a three-blade pulverizer.

FIG. 6a shows the TGA weight loss curve of PP/PP-MA-PEG wherein the peak decomposition temperature approached to 300° C., a value higher than that of the base PP plastic (277° C.). FIG. 6b similarly indicates the formation of the phase separated morphology of the as-extruded PP/PP-MA-PEG resin by exhibiting two endothermic peaks centered at about 57 and 137° C. which were respectively characteristic of PEG and PP constituents.

Embodiment 3

3.1 Single-Step Extrusion of PEG-Modified Maleated Olefin Bearing Polypropylene (PP/PP-MA-C/PEG)

PP/PP-MA-C/PEG was prepared by reactively extruding a dry blend mixture of three solid resin components: (1) PP (Total Petrochemicals Lumicene® MR10MX0), a random olefin copolymer; (2) PP-MA-C (Dow® Amplify™ GR 216), a maleic anhydride-grafted olefin plastomer and (3) PEG 10,000 (Tianjin Kermel). Prior to extrusion, 50 g of PEG 10,000 powders were pre-mixed together with 100 g of PP-MA-C pellets and 900 g of PP granules on the drum mixer. The dry blend mixture was fed into the twin-screw extruder from the front hopper. The barrel temperatures beginning from the front to the rear were 170° C., 180° C., 180° C., 180° C., 180° C., 180° C., 180° C. and 170° C. The feed frequency was 2 Hz while the speed of the screw was 150 rpm. The extrudate was cooled down in a water bath forming a solid filament and finally pelletized with a pelletizer. The pelletized resins were dried in a ventilated oven at 50° C. overnight.

FIG. 7 shows the ATR-FTIR spectrum of PP/PP-MA-C/PEG as compared with PP-MA-C. The melt flow index of PP/PP-MA-C/PEG was also measured to be 4.88 and 13.40 g/10 min respectively at 190 and 230° C. under the load of 2.16 kg, following the industrial standard of ASTM D1238-10. The results were higher than the values of 4.14 and 9.36 g/10 min recorded for the pristine PP (MR10MX0) under the same conditions. The increase of the melt flow index of PP after PEG modification is owing to the fact that PEG had a much lower melting point and molecular weight than PP. This result implicates that the PEG molecules tended to partition to melt surface during extrusion and therefore maintained the chance to reside PEG in the sample surface upon solidification.

Embodiment 4

4.1 Two-Step Fabrication of N-(4-Hydroxyphenyl)Maleimide-Modified PEG Bearing Polypropylene (PP/HPM-PEG) as Masterbatch

HPM-PEG, as an antifouling masterbatch precursor, was synthesized according to Equations (3)-(5). N-(4-hydroxyphenyl)maleimide (HPM) was prepared by addition of maleic anhydride (1.67M) and 4-aminophenol (1.67M) in dimethylformamide (DMF) followed by intramolecular condensation via a highly hygroscopic phosphorus pentoxide reagent (0.33M) and concentrated sulphuric acid (0.42M) leading to a closed ring maleimide group in a one-pot reaction. Hydroxyl end groups of a PEG 10,000 (Tianjin Kermel) chain were subsequently activated by tosylation in a 1:5 molar ratio to tosyl chloride to generate PEG-OTs in the presence of a base catalyst, such as triethylamine (Et₃N, 0.25M in dichloromethane). 0.025 M PEG-OTs and HPM in a 1:1 molar ratio were reacted under reflux in acetone in the presence of excess triethylamine (20:1 with respect to HPM) for at least two days. p-toluenesulfonic acid which was one major by-product formed insoluble acid-base adducts with triethylamine and precipitated out from the reaction solution. Finally, the acetone solution was filtered to remove the adduct by-products and then subjected to precipitation to yield HPM-PEG as deep orange powders in diethyl ether in the second step of filtration.

In the second step, prior to the extrusion process, 500 g PP (GD-150, Maoming Petro-Chemical Shihua Company), 2.5 g 2,5-bis(tert-butylperoxy)-2,5-dimethylhexane (Arkema Luperox® 101XL45), and 25 g HPM-PEG were pre-mixed on the rotating drum mixer by the same condition as in the foregoing sub-section 1.2, 1.3 or 1.4. Luperox® 101XL45, a heat-sensitive initiator, was added to generate free radicals by abstraction of hydrogen atoms from PP to allow additive coupling of PEG to PP via the maleimide group by extrusion. The reactive extrusion of the ternary blend was performed on the twin-screw extruder. The barrel temperature profile from the front to the rear was read as: 190° C., 200° C., 200° C., 200° C., 200° C., 190° C., 190° C., and 190° C. The feed frequency was 2 Hz and the speed of the screw was 150 rpm. The extrudate was solidified from melt upon cooling in water and finally subjected to pelletization. The plastic pellets were then dried in oven at 50° C. overnight to remove the absorbed moisture.

The melt flow index of the pristine PP (GD-150) was measured to be 9.68 and 43.6 g/10 min at 190° C. and 230° C. under the load of 2.16 kg. The melt flow index of PP/HPM-PEG resin was however not measurable even at 190° C. due to extremely high fluidity under the same load condition. The increase in melt flow index of the resin helps solve the mold filling problem by increasing the injection speed rather than increasing the processing temperature. A higher melt flow is recommended for injection molding in particular for thin-walled applications owing to the prerequisite of high shear rates being encountered. FIG. 8 is an ATR-FTIR spectrum of N-(4-hydroxyphenyl)maleimide-modified PEG bearing polypropylene (PP/HPM-PEG).

Embodiment 5

5.1 Bacterial Adsorption Studies on Antimicrobial Thermoplastic Resins and Masterbatches Thereof

Test specimens in circular discs with diameter and thickness of 64 mm and about 1 mm respectively were prepared by melting and fusion of the solid resin samples in a glass Petri dish on a heating plate. Solidification was done by cooling in atmospheric pressure at room temperature. To examine the bacterial adhesion and growth behavior on these pristine or modified specimen surfaces after thermoforming, swab test was achieved by collecting the adherent bacteria from the specimen surface using a cotton tip applicator (Medicom) prior to incubation with inoculums for the time elapsed for bioburden challenge test. Originally, plate counting of colonies along with serial dilution (in case of obtaining large colony populations) is used to determine the amount of viable bacteria in the inoculums after contact with products for a designated time at a given temperature. Slight modification on the protocol was made for executing the bacterial adsorption studies involving swabs. The tests were based on the original culture methodology but the amount of bacteria being attached to the product surface after contact was determined to assess the bacteria repellent performance of the plastic samples and their resistance towards bacterial colonization. In brief, a test inoculum of a selected Escherichia coli strain (ATCC® 8739™) was prepared and enumerated upon incubation according to the Japanese industrial standard (JIS Z 2801:2000) by finally adjusting the OD₆₀₀ of inoculum to 0.5 determined with a microplate reader (Molecular Devices SpectraMax M3). This corresponds to a population of approximately 10⁸ bacteria counts per millimeter of a 1/500 nutrient broth medium. Bioburden challenge procedure was subsequently carried out by incubating the inoculum of Escherichia coli (3 ml) over one face of a thermoformed plastic disc sample at 37° C. for 24 hours, followed by rinsing with 0.9% w/v saline for two to three times. The adherent bacteria remaining on the rinsed sample surfaces, which were inclined to biofilm growth, were swabbed and then dislodged to 1 millimeter of 0.9% w/v saline on a vortexer to perform conventional spread plating.

FIGS. 9 and 10 show the Escherichia coli colonies developed from the adherent species collected from several examples of antimicrobial masterbatches and resins deriving from PP and PC as the base plastics respectively: PP/PP-MA-C/PEG, PP/HPM-PEG, PP/SMA-PEG, PP/PP-MA-PEG and PC/SMA-PEG in comparison with their respective controls (pristine base plastics) without treatment. The bacterial adsorption studies were repeated once by using two separate test specimens for each sample. The complete set of spread count data is compiled in Table 1 which indicates that PP and PC after modifications with PEG-derived compounds or masterbatches by different methods demonstrated notable improvement of resistance towards Escherichia coli adsorption onto the sample surfaces.

Table 1 is a summary of plate count results of Escherichia coli adsorption on thermoformed specimens.

Dilution (Colony Forming Unit) Samples 10x 100x 1000x Resins with PP(MR10MX0) #1 >300 >300 41 low processing PP(MR10MX0) #2 >300 >300 46 temperature PP/PP-MA-C/PEG #1 148 15 1 PP/PP-MA-C/PEG #2 >300 65 4 PP/PP-MA-PEG #1 15 1 0 PP/PP-MA-PEG #2 0 0 0 PP(GD-150) #1 >300 >300 34 PP(GD-150) #2 >300 >300 44 PP/SMA-PEG #1 0 0 0 PP/SMA-PEG #2 0 0 0 PP/HPM-PEG #1 0 0 0 PP/HPM-PEG #2 0 0 0 Resins with PC #1 >300 >300 >300 high processing PC #2 >300 >300 197 temperature PC/SMA-PEG #1 10 0 0 PC/SMA-PEG #2 12 0 0

5.2 Bacterial Adsorption Studies on Specimens Injection-Molded from Antimicrobial PE Resins (PE/SMA-PEG)

The pristine PE and PE/SMA-PEG resins were injection-molded into plastic circular dishes with a diameter and height of 50 mm and 15 mm respectively via a desktop vertical plunger-type injection molding machine (Model AB-400M, A.B.Machinery, Canada). The barrel temperature was 200° C. The pressure was 60 psi.

The bacterial adsorption studies were performed to directly observe the Escherichia coli attached on the surface of the sample.

10 mL of diluted Escherichia coli suspension in 1/500 nutrient broth medium with a cell density of approximately 10⁴ cells/mL was added into the injection-molded plastic dishes injection-molded from PE/SMA-PEG. The dishes were directly incubated against the bacterial suspension at 37° C. for 24 hours, followed by rinsing with 0.9% w/v saline for two to three times. 6 mL of nutrient agar solution at about 50° C. was poured to the dishes and further incubated overnight at 37° C. The agar solution gradually solidified as nutrient source and those viable bacteria which had adhered strongly to the dish surface developed into colonies at the bottom of the plastic dishes. FIG. 11 demonstrates the development of colonies between PE/SMA-PEG and the pristine PE without treatment. Colonies developed only on the dish injection-molded from the pristine PE meaning that the specimens injection-molded from PE/SMA-PEG resins was able to prevent initial inhabitation of bacteria.

Embodiment 6

6.1 Single-Step Extrusion of PEG-Containing Polypropylene in the Presence of Styrene-Maleic Anhydride Copolymer and Citric Acid (PP/SMA/PEG/CA)

A dry blend mixture of the following solid components and additives: (1) PP (Total Petrochemicals Lumicene® MR10MX0), a random olefin copolymer; (2) SMA (Sigma-Aldrich, Catalog no. 442399), a styrene-maleic anhydride copolymer; (3) PEG 10,000 (Aoki Oil Industrial Co., Ltd.) and (4) CA, anhydrous citric acid (Acros, Organics, Manufacturer part no. 423565000). The extrusion was undergone on a co-rotating twin screw extruder (Model TE-35, Jiangsu (Nanjing) Keya Co. China) with a screw diameter of 35.6 mm, a length-to-diameter ratio of about 45 and a maximum screw speed of 600 rpm as driven by an 11-kW motor. A mixture of 200 g of PEG 10,000, 6 g of SMA, 40 g of citric acid and 4 kg of PP (MR10MX0) granules was fed into the extruder from the main hopper. The barrel temperatures beginning from the front to the rear (with a total of 8 temperature zones) were 190° C., 200° C., 210° C., 210° C., 200° C., 200° C., 200° C. and 200° C. The feed frequency was 5 Hz and the screw speed was 300 rpm. Pelletized resins were dried in a hot air dryer at 70° C. for 3 hours before use.

Table 2 is a summary of the physical properties of PP/SMA/PEG/CA, demonstrating minimal effects of antimicrobial treatment on the properties of the pristine PP.

Properties PP (MR10MX0) PP/SMA/PEG/CA Tensile strength¹/MPa 30 25 Optical transmission ²/% 85 80 Volume resistivity ³/Ω · cm 7.4 × 10¹¹ 8.0 × 10¹² Dielectric strength ⁴/kV mm⁻¹ 40 35 Thermal conductivity ⁵/W m⁻¹ k⁻¹ 0.2 0.2 ¹Per ASTM 0838: Type V^(c) specimens, injection-molded, tensile speed 100 mm/min, 23 ± 2° C. ) ² Per ASTM D1003: thickness 1.3 mm (0.05 in) ³ Per ANSI/ESD STM11.12: one face of a hot-pressed film spray-coated with graphite as electrode ⁴ Per ASTM D149-09 (Method A): one face of a hot-pressed film spray-coated with graphite as electrode ⁵ Measured with a light flash apparatus (NETZSCH LFA 467 HyperFlash ® model) on injection-molded circular discs (diameter 12.7 mm; thickness 2.0 mm) with silver coating (100 nm) on flat surfaces by vacuum evaporation

Plastic jelly cups were prepared from PP/SMA/PEG/CA using a 100-ton injection molding machine (Cosmos TTI-model, Welltec Industrial Equipment Ltd.). The jelly cups passed the chemical migration tests performed in an accredited agency. The tests comply with national regulations, including EU No. 10/2011 (overall migration against both 3% w/v acetic acid and 20% v/v ethanol as food stimulants at 20° C. for 6 hours and US FDA 21 CFR 177.1520(c), Items 3.1a and 3.2a as a polypropylene copolymer for intended uses in food contact articles). The jelly cups also comply with the specific migration limits of the heavy metals (barium, cobalt, copper, iron, lithium, manganese and zinc) and primary aromatic amines for the specific stimulant used (3% w/v acetic acid) at 20° C. for 6 hours as set out by EU No. 10/2011.

6.2 Bacterial Adsorption and Biocompatibility Studies on Hot-Pressed Film Specimens from Antimicrobial PP Resins (PP/SMA/PEG/CA)

To carry out bacteria adsorption and biocompatibility tests on the hot-pressed film samples of PP/SMA/PEG/CA, a test inoculum of a selected Escherichia coli strain (ATCC® 8739™), a Gram-negative and Staphylococcus aureus strain (ATCC® 6538™) a Gram-positive, were prepared and enumerated upon incubation by finally adjusting the OD₆₀₀ of inoculum to 0.6 and 1.5 respectively and then diluted by 10 times and 500 times using 1/500 nutrient broth medium. These resulted in a bacterial population of approximately 1×10⁷ and 2×10⁵ cells/mL. Bioburden challenge was subsequently performed by inoculating either Escherichia coli or Staphylococcus aureus (2 mL each) over one face of three separate film specimens at 37° C. for 24 hours, followed by rinsing with 0.9% w/v saline for two to three times. The inoculated surface was swabbed after 24 hours and then dislodged to 1 mL of 0.9% w/v saline on a vortexer. Plating of the bacterial suspension was rendered with a spiral plater (Eddy Jet 2, IUL Instruments) coupled to an image analyzer (Flash & Go Colony Counter, IUL Instruments), avoiding multiple dilution procedures.

Table 3 is a summary of bacterial counts expressed in terms of the unit of colony forming units (CFU) per mL against spiking of high concentration of Escherichia coli and Staphylococcus aureus.

Bacteria Sample/CFU mL⁻¹ #1 #2 #3 Average Escherichia coli PP (MR10MX0) 9.40 × 10³ 1.55 × 10³ 1.11 × 10⁴ 7.35 × 10³ PP/SMA/PEG/CA 1.02 × 10² 6.10 × 10¹ 4.07 × 10¹ 6.79 × 10¹ Staphylococcus PP (MR10MX0) 4.64 × 10⁴ 2.22 × 10² 8.07 × 10¹ 2.56 × 10³ aureus PP/SMA/PEG/CA 6.10 × 10¹ 6.10 × 10¹ 6.10 × 10¹ 6.10 × 10¹

Biocompatibility of the film specimens hot-pressed from antimicrobial PP resins was tested by Cell Counting Kit (CCK)-8 colorimetric assay (Dojindo Molecular Technologies) for in-vitro cytotoxicity assessment in accordance with the minimum essential medium (MEM) elution methodology described by the ISO 10993-5 standard as described in FIG. 13.

Slight modification of the protocol was done by choosing a different cell line to ensure that the antimicrobial PP (PP/SMA/PEG/CA) is safe on human skin contact. The selected human cell line was HaCaT (CLS Cell Lines Service GmbH, Germany), an epidermis cell type (keratinocyte). Three films each of the antimicrobial PP and the pristine PP were prepared for cytotoxicity analyses. The cell viability in the medium was set to be 100%. FIG. 12 indicates that after 24 and 48 hours of incubation against the test pieces, the antimicrobial PP was non-cytotoxic to HaCaT cells as in the case of the pristine PP showing more than 90% of cell viability.

6.3 Bacterial Adsorption Studies on Hot-Pressed Film Specimens from Antimicrobial PVC Resins (PVC/SMA/PEG/CA)

Almost equivalent formulation of PP/SMA/PEG/CA in the foregoing sub-sections 6.1 and 6.2 was exploited for flexible PVC (LG Chem HB-65) as the base plastic for PVC/SMA/PEG/CA. In brief, 100 g of flexible PVC granules were pre-mixed with 5 g of PEG 600 (acquired from Shanghai Zhanyun Chemical Co., Ltd.), 0.15 g of SMA and 1 g of CA and then extruded from a single-screw extruder (Wellzoom C-type) at the temperature settings of 160° C. (barrel) and 165° C. (die) and a screw speed of about 10 rpm. Pelletized resins were consequently dried at 50° C. overnight before use. One face of the hot-pressed film were similarly challenged against Staphylococcus aureus at 37° C. for 24 hours as in the case of PP/SMA/PEG/CA, followed by rinsing with 0.9% w/v saline for two to three times. The inoculated surface was swabbed after 24 hours and then dislodged to 1 mL of 0.9% w/v saline on a vortexer. Counting was then done with the aid of a spiral plater and an image analyzer.

Table 4 is a summary of adsorbed bacterial counts after bioburden challenge against Staphylococcus aureus that the surfaces of antimicrobial PVC were completely repellent against the bacteria.

Sample/CFU · mL⁻¹ #1 #2 #3 Average PVC (HB-65) 4.33 × 10³ 1.76 × 10⁴ 1.41 × 10⁵ 5.43 × 10⁴ PVC/SMA/FEG/CA 0 0 0 0 

What is claimed is:
 1. A method for preparing an antimicrobial thermoplastic resin, the method comprising the steps of: preparing an antimicrobial thermoplastic resin by melt extruding a composition comprising an intermediate, polyethylene glycol (PEG), and a basic plastic selected from the group consisting of a polyolefin, polyvinyl chloride, and a polycarbonate, wherein said antimicrobial thermoplastic is biocide-free and has bacterial-repellent properties and the intermediate is selected from the group consisting of: a co-polymer comprising a polystyrene repeating unit and a maleic anhydride repeating unit; a maleic anhydride grafted polypropylene; and a maleic anhydride-grafted olefin plastomer.
 2. The method of claim 1 further comprising the step of molding the antimicrobial thermoplastic resin into a finished article through a thermoforming process.
 3. The method of claim 2, wherein the finished article is in a form selected from the group consisting of solid, monolith, tube, composite, fiber, film, sheet and varnish.
 4. The method of claim 1, wherein the basic plastic is a thermoplastic and melt-processable plastic resin selected from the group consisting polyethylene, polypropylene, and polyvinyl chloride.
 5. The method of claim 1, wherein the composition further comprises citric acid.
 6. The method of claim 1, wherein the intermediate is a co-polymer comprising a polystyrene repeating unit and a maleic anhydride repeating unit.
 7. The method of claim 6, wherein the basic plastic is selected from the group consisting polyethylene, polypropylene, and polyvinyl chloride.
 8. The method of claim 6, wherein the basic plastic is selected from the group consisting polypropylene and polyvinyl chloride.
 9. The method of claim 8, wherein the basic plastic is polypropylene and PEG, the intermediate, and polypropylene are present in the antimicrobial thermoplastic resin in a mass ratio of 33:1:667, respectively.
 10. The method of claim 8, wherein the basic plastic is polyvinyl chloride and the PEG, the intermediate, and polyvinyl chloride are present in the antimicrobial thermoplastic resin in a mass ratio of 33:1:667, respectively.
 11. The method of claim 8, wherein the composition further comprises citric acid.
 12. The method of claim 1, wherein the intermediate is a maleic anhydride-grafted olefin plastomer.
 13. The method of claim 12, wherein the basic plastic is polypropylene.
 14. The method of claim 13, wherein PEG, the intermediate, and polypropylene are present in the antimicrobial thermoplastic resin in a mass ratio of 1:2:18, respectively.
 15. An antimicrobial thermoplastic resin prepared by the method of claim
 1. 16. An antimicrobial thermoplastic resin prepared by the method of claim
 8. 17. An antimicrobial thermoplastic resin prepared by the method of claim
 9. 18. An antimicrobial thermoplastic resin prepared by the method of claim
 10. 19. An antimicrobial thermoplastic resin prepared by the method of claim
 13. 20. An antimicrobial thermoplastic resin prepared by the method of claim
 14. 